John Roberts, the chief justice of the United States, deals with more than just dull legal arguments. He also is a grammarian. He demonstrated this additional skill in the recent case AT&T v. Federal Communications Commission.

AT&T had insisted that it could refuse to produce certain documents. The FCC’s statute on Freedom of Information grants exemption if a disclosure “could reasonably be expected to constitute an unwarranted invasion of personal privacy.” The Congress has defined “person” to include a corporation, so AT&T was exempt. Right?

The U.S. Court of Appeals, Third Circuit, agreed. It said, “It would be very odd indeed for an adjectival form of a defined term not to refer back to that defined term.”

The appellate court noted that another FCC exemption protected information that, if released, “could reasonably be expected to endanger the life or physical safety of any individual.” Thus, Congress showed that it knew how to refer solely to human beings and exclude corporations when it wanted to.

Enter Chief Justice Roberts. He saw to it that the decision would be unanimous, except for Justice Elena Kagan, who abstained since she had worked on the case when she was the U.S. solicitor general. He wrote the decision.

He pointedly disagreed with AT&T’s contention that, since Congress had defined the noun “person” to include corporations, “it necessarily defined the adjective form of that noun — ‘personal’ — also to include corporations.” He italicized the word “necessarily.”

The chief justice wrote: “Adjectives typically reflect the meaning of corresponding nouns, but not always. Sometimes they acquire distinct meanings of their own.” He wrote that the noun “crab” refers variously to a crustacean and a type of apple, while the related adjective “crabbed” can refer to handwriting that is difficult to read. He went on to contrast “corn” with “corny” and “crank” with “cranky.”

Returning to “personal,” he said that word ordinarily refers to individuals: “We do not usually speak of personal characteristics, personal effects, personal correspondence, personal influence, or personal tragedy as referring to corporations. This is not to say that corporations do not have correspondence, influence, or tragedies of their own, only that we do not use the word ‘personal’ to describe them.”